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Mexico will want it back.
They’ll (edit: we’ll) potentially want the Rio Grande valley back (plus San Antone, South Padre, Austin...). The rest, may I suggest a wall?
^ the Rio Grande valley and its outlet malls is oft-frequented by Mexicans for shopping; South Padre is a popular beach destination for northeastern Mexicans; Austin is cool; San Antonio is nice, has good shopping, and is conveniently already full of Mexicans.
Mexico will want it back.